A HIFU excitation scheme to reduce switching-induced grating lobes and hard tissue interface heating

Arbitrary excitation waveforms are desirable in HIFU array systems. In trans-skull therapy this is necessary to compensate for phase aberrations and achieve even heating of the skull. To facilitate this, wave form generators and large, costly class A RF amplifiers are used. High element counts are commonplace and so the portability of these systems is greatly reduced. We propose using switched mode excitation to miniaturise these systems. Unfortunately, the curvature of therapeutic arrays and the inherent harmonic content of switched circuits induces harmful grating lobes into the therapeutic field of view. The advent of higher bandwidth transducers makes harmonic cancellation imperative. 3rd harmonic reduction (3HR) PWM can be used to negate these grating lobes but cannot implement amplitude control. We propose the use of the HRPWM technique to negate switching induced grating lobes and modulate output power to reduce hard tissue heating.